A solar cell converts solar energy directly to DC electric energy. Generally configured as a photodiode, it permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier (electrons or holes (a lack of electrons)) may be extracted as current. And like most other diodes, photodiodes are formed by combining p-type and n-type semiconductors to form a junction.
Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
Substantially affecting solar cell performance, carrier lifetime (recombination lifetime) is defined as the average time it takes an excess minority carrier (non-dominant current carrier in a semiconductor region) to recombine and thus become unavailable to conduct an electrical current. Likewise, diffusion length is the average distance that a charge carrier travels before it recombines. In general, although increasing dopant concentration improves conductivity, it also tends to increase recombination. Consequently, the shorter the recombination lifetime or recombination length, the closer the metal region must be to where the charge carrier was generated.
Most solar cells are generally formed on a silicon substrate doped with a first dopant (commonly boron) forming an absorber region, upon which a second counter dopant (commonly phosphorous), is diffused forming the emitter region, in order to complete the p-n junction. After the addition of passivation, back surface field (BSF), and anti-reflection coatings, metal contacts (fingers and busbar on the emitter and pads on the back of the absorber) may be added in order to extract generated charge. The BSF, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
For example, aluminum deposited on the rear of the solar cell and heated at temperatures between 700° C. and 1000° C., forms a BSF that is a combination of P—Si/P+—Si/Si—Al eutectic and agglomerated Al.
Referring now to FIG. 1, a simplified diagram of a traditional front-contact solar cell is shown. In a common configuration, a phosphorous-doped (n-type) emitter region 108 is first formed on a boron-doped silicon substrate 110 (p-type), although a configuration with a boron-doped emitter region on a phosphorus-doped silicon substrate may also be used.
Prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, residual surface glass (PSG) formed on the substrate surface during the POCl3 deposition process may be removed by exposing the doped silicon substrate to an etchant, such as hydrofluoric acid (HF). The set of metal contacts, comprising front-metal contact 102 and back surface field (BSF)/back metal contact 116, are then sequentially formed on and subsequently fired into doped silicon substrate 110.
The front metal contact 102 is commonly formed by depositing an Ag (silver) paste, comprising Ag powder (about 70 to about 80 wt % (weight percent)), lead borosilicate glass (frit) PbO—B2O3—SiO2 (about 1 to about 10 wt %), and organic components (about 15 to about 30 wt %). After deposition the paste is dried at a low temperature to remove organic solvents and fired at high temperatures to form the conductive metal layer and to enable the silicon-metal contact.
BSF/back metal contact 116 is generally formed from aluminum (in the case of a p-type substrate) and is configured to create a potential barrier that repels and thus minimizes the impact of minority carrier rear surface recombination. In addition, Ag pads [not shown] are generally applied onto BSF/back metal contract 116 in order to facilitate soldering for interconnection into modules.
However, the use of an Al-BSF may also be problematic. Al-BSF tends to cause solar cell warping, which leads to difficulties in subsequent production processes and decreases the yield due to increased breakage. In addition, not only is Al-BSF a suboptimal reflection surface, reducing the red spectrum that would otherwise be reflected back into the wafer substrate, but it is also not generally the best form of rear passivation available.
One solution is to replace the full area Al-BSF with a more reflective better passivated layer and make contact to the bulk through reduced area metal contacts. A solar cell configured in such a way will reduce charge carrier recombination in the bulk and increase absorption of long wavelength light. Solar cells configured with this architecture are commonly referred to as PERC (Passivated Emitter and Rear Cell) an architecture that was first introduced in 1989 by the University of New South Wales [A. W. Blakers, et al., Applied Physics Letters, 55 (1989) 1363-1365]. The devices fabricated in that study used heavily doped substrates as well as numerous expensive processing steps that are not compatible with high throughput manufacturing. Other versions of this cell architecture were later introduced as options to further increase the efficiency. Most notable among them is the PERL (passivated emitter rear locally diffused) [A. Wang, et al. J. Appl. Phys. Lett. 57, 602, (1990)] and PERF (passivated emitter rear floating junction) cells [P. P. Altermatt, et al. J. Appl. Phys. 80 (6), September 1996, pp. 3574-3586]. Similar to the original PERC cell, these architectures are expensive to manufacture. Since their introduction there have been numerous attempts to develop an industrially viable approach to make these cells.
One important feature of these cells is the passivation layer on the rear surface. One approach is to use the residual rear phosphorous diffusion, created during the front-side phosphorous diffusion process (or in a separate diffusion step), provided it is disconnected from the front junction. This type of passivation is referred to as a rear floating junction and has been shown to provide excellent quality rear passivation [C. B. Honsberg, Solar Energy Materials and Solar Cells 34, Issues 1-4, 1 Sep. 1994, Pages 117-123]. Another type of rear floating junction can be formed by removing the rear n-type diffusion and passivating the bulk silicon with a silicon nitride layer. In this case the fixed charge in the nitride causes an inversion layer to form, resulting in an effective floating junction.
Floating junctions provide excellent rear surface passivation, but they do not allow a contact to be formed between the rear metal electrode and the bulk of the wafer. The cell design requires that selective openings be formed in the rear surface passivation layer through which metal is able to contact to the bulk region. When this happens it is very common for the metallization used to form a linear shunt between the floating junction and the substrate. This shunt path greatly reduces the passivation provided by the floating junction, resulting in reduced cell efficiency [S. Dauwe, et al. Prog. Photovolt: Res. Appl. 2002; 10:271-278].
Referring now to FIG. 2, a simplified diagram of a passivated layer/reduced area metal contact solar cell configuration with detrimental shunting. An ohmic contact is necessary for a good silicon metal interface. In general, increasing the doping concentration of the silicon allows formation of an ohmic contact.
Metal contact 216, (comprising Ag paste with a lead borosilicate glass frit or Al BSF paste), is generally fired at high temperatures through rear passivation/reflectivity layer 214 (commonly SiNx) to form the conductive silicon-metal contact with p-type silicon substrate 210. The region under the metal is heavily doped p-type 208 to facilitate good ohmic contact to the silicon bulk 210. This doping may be formed before metal firing or during firing such as the case with Al BSF paste. This region must also separate the metal 216 from the floating junction passivation 212 to avoid the aforementioned shunting problem. If it is unable to separate the metal from the floating junction, for example because of a substantially greater dopant concentration in the floating junction (5e19 cm−3 to 1e21 cm−3) compared to the p+ region formed with Al BSF paste (<1e19 cm−3), then a shunt path will form and the rear passivation will not function correctly 218.
In view of the foregoing, there is a desire to form a floating junction on a solar cell with a particle masking layer.